ONU 



OPA 



radical acetyl with hydrogen. Its name 

 was derived from its forming an oily 

 substance with chlorine. 



OLEIC ACID {oleum, oil). An acid 

 forming the essential part of fat oils 

 which are not drying, as oil of almonds. 



OLEO-RESINS. Native compounds 

 of volatile oil and resin, the proper juices 

 of coniferous and other plants. 



O'LIGIST IRON. A subspecies of 

 rhomboidal iron ore, comprising the com- 

 mon specular, and the micaceous spe- 

 cular varieties. 



OLIGO- (o\i7o?, little, few). A term 

 used in Greek compounds, to denote that 

 the number of any thing is small, not 

 indefinite. It is contrasted by the pre- 

 fix poly- (ttoAus, many), signifying that 

 the number is large and not definite. 

 Thus we have o/i^'o-spermous &nd poly- 

 spermous fruits. 



OLIVE MALACHITE. Octahedral 

 phosphate of copper, from Lebethen in 

 Hungary. 



OLIVE ORE. Olivenite. One of the 

 arseniates of copper. 



OLIVILE {olea, the olive). The 

 name given by Pelletier to a peculiar 

 substance which remains after gently 

 evaporating the alcoholic solution of 

 the gum which exudes from the olive 

 tree. 



OLI'VIN^. An aberrant sub-family 

 of the VolutidcB, or volutes, named from 

 the typical genus oliva ; these shells are 

 cylindrical in shape ; the aperture is 

 linear, and the pillar thickened and con- 

 fusedly plaited. 



O'LIVINE {olea, the olive). An olive- 

 coloured silicate of lime and magnesia. 

 In its purer state it is denomfhated 

 chrysolite or peridot; and when protox- 

 ide of iron is predominant, it has by 

 some been called hyalosiderite. 



OMNI'VOROUS {omnis, all, voro, to 

 devour). A term applied to animals 

 which feed on all substances indiflferently. 

 The term is synonymous with the Greek 

 pantophagovs. 



O'MPHaCITE. a variety of augite, 

 of a pale green colour, occurring in primi- 

 tive rocks, with precious garnet, in Ca- 

 rinthia. 



OMPHALO'DIUM (iM0a\69, the um- 

 bilicus). A term applied by Turpin to 

 the centre of the hilum of the seeds of 

 plants, through which the nutrient ves- 

 sels pass to the embryo. 



ONUS PROBANDI. The burden of 

 proof, in Rhetoric, rests with him who 

 would deny any generally received 

 241 



opinion, as that of gravitation, that of 

 the Copernican theory, &c. 



O'NYX {ovv^, onyx, a nail). A variety 

 of agate, in which the siliceous particles 

 are disposed in alternating horizontal 

 layers of opaque white, and translucent 

 blue, grey, or brown, resembling the 

 marks on the human nail. 



O'OLITE {^6v, an egg, XiOo^, a stone). 

 A limestone ; so named from its being 

 composed of rounded particles, like the 

 roe or eggs of a fish. The term is also 

 applied to a large group of strata, charac- 

 terized by peculiar fossils, in which 

 limestone of this texture occurs. 



1. Lower Oolite. A series of strata of 

 oolitic limestone, clays, and marls, re- 

 posing above the lias, and divisible into 

 the Lower or Cheltenham Oolite, Fuller's 

 Earth, Stonesfield Si ate, Great Oolite, 

 Ferest Marble, and Corn-brash. 



2. Middle Oolite. A series of strata, 

 consisting of coralline and shelly oolitic 

 limestone, calcareous sandstone, and 

 clays, separated from the Lower Oolite 

 by a great deposit of clay, and divisible 

 into Oxford Clay, Calcareous Grit or 

 Sandstone, Coral Rag and Oolite, and 

 Calcareous Sandstone. 



3. Upper Oolite. A series of strata 

 separated from the Middle Oolite by a 

 great deposit of clay, and divisible into 

 the Kimmeridge Clay, Portland Sand, 

 and Portland Oolite. 



OOZO'A {ujov, an egg, ^wov, an ani- 

 mal). A designation given by Cams to 

 one of the primary divisions of the Ani- 

 mal Kingdom, from the resemblance of 

 the beings composing it to the eggs or 

 rudiments of more perfect forms. They 

 correspond wih the Acrita of Macleay, 

 the Cryptoneura of Rudolphi, and the 

 Protozoa of other writers. 



OPA'CITY {opacus, dark). A quality 

 of bodies which renders them impervious 

 to the rays of light. In this respect 

 opaque bodies are the reverse of the 

 transparent ; but their opacity is occa- 

 sioned rather by their thickness than 

 their absolute impermeability to light. 

 Gold is considered opaque ; but the rays 

 of a candle, or of the sun, falling on a 

 sheet of gold leaf laid on glass, are trans- 

 mitted as a faint green glimmering light. 



OPAL. Opaline substances are mostly 

 hydrates of silica, and are named — 



1. Precious ox Noble Opal, which owes 

 its beautiful play of colours to a multi- 

 plicity of imperceptible fissures in its 

 interior. A variety of this kind has the 

 property of becoming transparent when 

 M 



